U.S. patent number 4,937,526 [Application Number 07/365,632] was granted by the patent office on 1990-06-26 for adaptive method for reducing motion and flow artifacts in nmr images.
This patent grant is currently assigned to Mayo Foundation for Medical Education and Research. Invention is credited to Richard L. Ehman, Joel P. Felmlee.
United States Patent |
4,937,526 |
Ehman , et al. |
June 26, 1990 |
Adaptive method for reducing motion and flow artifacts in NMR
images
Abstract
An NMR data set which is acquired with applied phase encoding
and read-out gradients is corrected to reduce motion artifacts and
increase image sharpness. The acquired NMR data set is examined to
detect bulk displacements of the object being imaged and phase
displacements caused by motion. This information is employed to
produce correction operators which are applied to the NMR image
data set. One or more navigator signals may also be acquired during
the scan to produce an NMR data set from which the corrective
operators can more readily be derived.
Inventors: |
Ehman; Richard L. (Rochester,
MN), Felmlee; Joel P. (Rochester, MN) |
Assignee: |
Mayo Foundation for Medical
Education and Research (Rochester, MN)
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Family
ID: |
26957819 |
Appl.
No.: |
07/365,632 |
Filed: |
June 13, 1989 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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276168 |
Nov 23, 1988 |
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Current U.S.
Class: |
324/309;
324/306 |
Current CPC
Class: |
G01R
33/5676 (20130101) |
Current International
Class: |
G01R
33/567 (20060101); G01R 33/54 (20060101); G01R
033/20 () |
Field of
Search: |
;324/300,306,307,309,318,322 ;128/653 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Haacke, Mark E. and Patrick, John L. "Reducing Motion Artifacts in
Two-Dimensional Fourier Transform Imaging," Magnetic Resonance
Imaging, vol. 4, pp. 359-376, 1986. .
Cuppen, et al. "Reduction of Motion Artifacts by Data
Postprocessing," Book of Abstracts, Fourth Annual Meeting, Aug.
19-23, 1985. .
"The Effect of Motion on Two-Dimensional Fourier Transformation
Magnetic Resonance Images", C. L. Schultz et al., Radiology 1984;
152:117-121. .
"Influence of Physiologic Motion on the Appearance of Tissue in MR
Images", R. L. Ehman et al., Radiology 1986; 159:777-782. .
"Respiratory Effects in Two-Dimensional Fourier Transform MR
Imaging", L. Axel et al., Radiology 1986; 160:795-801. .
"The Magnetic Field Dependence of the Breathing Artifact", M. L.
Wood et al., Mag. Res. Imag. 1986; 4:387-392. .
"Suppression of Respiratory Motion Artifacts in Magnetic Resonance
Imaging", M. L. Wood et al., Med. Phys. 1986; 13(6):794-805. .
"Instant Images of the Human Heart Using a New, Whole Body MR
Imaging System", R. R. Rzedzian et al., AJR 1986; 149:245-250.
.
"Motion Artifact Reduction with Fast Spin-Echo Imaging", D. D.
Stark et al., Radiology 1987; 164:183-191. .
"Cardiac Imaging Using Gated Magnetic Resonance", P. Lanzer et al.,
Radiology 1984; 150:121-127. .
"Magnetic Resonance Imaging with Respiratory Gating: Techniques and
Advantages", R. L. Ehman et al., AJR 1984; 143:1175-1182. .
"Respiratory Ordered Phase Encoding (ROPE): A Method for Reducing
Respiratory Motion Artifacts in MR Imaging", D. R. Bailes et al.,
Jour. Comp. Asst. Tomo. 1985; 9(4):835-838. .
"Motion Artifact Suppression Technique (MAST) for MR Imaging", P.
M. Pattany et al., Jour. Comp. Asst. Tomo. 1987; 11(3):369-377.
.
"Spatial Presaturation: A Method for Suppressing Flow Artifacts and
Improving Depiction of Vascular Anatomy in MR Imaging", J. P.
Felmlee et al., Radiology 1987; 164:559-564. .
"Frodo Pulse Sequences: A New Means of Eliminating Motion, Flow,
and Wraparound Artifacts", R. R. Edelman et al., Radiology 1988;
166:231-236. .
"Craniocaudal Movements of the Liver, Pancreas, and Kidneys in
Respiration", L. Suramo et al., Acta Radiol (Diagn) (Stockh) 1984;
25:129-131. .
"Monitored Echo Gating (Mega) for the Reduction of Motion
Artifacts", R. S. Hinks, presented at the meeting of the Society
for Magnetic Resonance Imaging, Feb. 28, 1988, #107, p. 48. .
The Fourier Transform and Its Applications, R. N. Bracewell,
McGraw-Hill Book Co., 2nd Ed. 1978, p. 104. .
"Effects of Diffusion on Free Precession in Nuclear Magnetic
Resonance Experiments", H. Y. Carr et al., Phys. Rev. 1954;
4:630-8. .
"Verification and Evaluation of Internal Flow and Motion", P. R.
Moran et al., Radiology 1985; 154:433-441. .
"Direct NMR Imaging of Heart Wall and Blood Flow Velocity", P. Van
Dijk, Jour. Comp. Asst. Tomo. 1984; 8:429-436. .
"Blood Flow Imaging with MR: Spin-Phase Phenomena", G. K. Von
Schulthess et al., Radiology 1985; 157:687-695. .
"Blood Flow Effects in Magnetic Resonance Imaging", L. Axel, AJR
1984; 143:1157-1166. .
"Blood Flow: Magnetic Resonance Imaging", W. G. Bradley et al.,
Radiology 1985; 154:443-450..
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Primary Examiner: Tokar; Michael J.
Attorney, Agent or Firm: Quarles & Brady
Parent Case Text
CROSS REFERENCE
This application is a continuation-in-part of application Ser. No.
07/276,168, filed Nov. 23, 1988 now abandoned.
Claims
We claim:
1. An NMR system, the combination comprising:
means for generating a polarizing magnetic field;
excitation means for generating an RF excitation magnetic field
which produces transverse magnetization in nuclei subjected to the
polarizing magnetic field;
receiver means for sensing the NMR signal produced by the
transverse magnetization and producing digitized in-phase (I) and
quadrature (Q) samples of the NMR signal;
first gradient means for generating a first magnetic field gradient
to impart a first phase component into the NMR signal which is
indicative of the location of the transversely magnetized nuclei
along a first coordinate axis;
second gradient means for generating a second magnetic field
gradient to impart a second phase component into the NMR signal
which is indicative of the location of the transversely magnetized
nuclei along a second coordinate axis;
pulse control means coupled to the excitation means, first and
second gradient means, and receiver means, said pulse control means
being operable to conduct a scan in which a series of pulse
sequences are conducted in which the second magnetic field gradient
is stepped through a series of discrete values and a corresponding
series of NMR signals are sensed and digitized to form an NMR data
set; and
processor means for storing the NMR data set and for reconstructing
an image array for a display from the stored NMR data set by:
(a) Fourier transforming the NMR data set along one of its
dimensions to create hybrid-space data arrays I' and Q';
(b) producing a correction data array using the data in the
hybrid-space data arrays I' and Q' by calculating the magnitude of
the transformed sampled NMR signals in the hybrid-space data arrays
I' and Q' to produce a modulus array, and correlating each row of
the modulus array to produce a corresponding shift value for the
correction data array;
(c) applying the data in the correction data array to the NMR data
set to reduce motion effects; and
(d) Fourier transforming the corrected NMR image data set to
produce the image array.
2. The NMR system as recited in claim 1 in which a rollback
correction data array is produced from the corresponding shift
values in the correction data array and a rollover rate K.sub.R,
and the rollback correction data array values are applied to
correct the NMR image data set in step (c).
3. An NMR system, the combination comprising:
means for generating a polarizing magnetic field;
excitation means for generating an RF excitation magnetic field
which produces transverse magnetization in nuclei subjected to the
polarizing magnetic field;
receiver means for sensing the NMR signal produced by the
transverse magnetization and producing digitized in-phase (I) and
quadrature (Q) samples of the NMR signal;
first gradient means for generating a first magnetic field gradient
to impart a first phase component into the NMR signal which is
indicative of the location of the transversely magnetized nuclei
along a first coordinate axis;
second gradient means for generating a second magnetic field
gradient to impart a second phase component into the NMR signal
which is indicative of the location of the transversely magnetized
nuclei along a second coordinate axis;
pulse control means coupled to the excitation means, first and
second gradient means, and receiver means, said pulse control means
being operable to conduct a scan in which a series of pulse
sequences are conducted in which the second magnetic field gradient
is stepped through a series of discrete values and a corresponding
series of NMR signals are sensed and digitized to form an NMR data
set; and
processor means for storing the NMR data set and for reconstructing
an image array for a display from the stored NMR data set by:
(a) Fourier transforming the NMR data set along one of its
dimensions to create hybrid-space data arrays I' and Q';
(b) producing a correction data array using the data in the
hybrid-space data arrays I' and Q' by calculating the phase of the
transformed sampled NMR signals in the hybrid-space data arrays I'
and Q' to produce a two-dimensional phase array, and determining
the difference in phase between elements in a reference row of the
phase array and elements in the same column of the phase array to
produce the values for the correction data array;
(c) applying the data in the correction data array to the NMR data
set to reduce motion effects; and
(d) Fourier transforming the corrected NMR image data set to
produce the image array.
4. The NMR system as recited in claim 1 in which a second
correction data array is produced by calculating the phase of the
transformed sampled NMR signals in the hybrid-space data arrays I'
and Q' to produce a two-dimensional phase array, and determining
the phase difference between elements in a reference row of the
phase array and elements in the same column of the phase array to
produce the values for the second correction data array.
5. An NMR system, the combination comprising:
means for generating a polarizing magnetic field;
excitation means for generating an RF excitation magnetic field
which produces transverse magnetization in nuclei subjected to the
polarizing magnetic field;
receiver means for sensing the NMR signal produced by the
transverse magnetization and producing digitized in-phase (I) and
quadrature (Q) samples of the NMR signal;
first gradient means for generating a first magnetic field gradient
to impart a first phase component into the NMR signal which is
indicative of the location of the transversely magnetized nuclei
along a first coordinate axis;
second gradient means for generating a second magnetic field
gradient to impart a second phase component into the NMR signal
which is indicative of the location of the transversely magnetized
nuclei along a second coordinate axis;
pulse control means coupled to the excitation means, first and
second gradient means, and receiver means, said pulse control means
being operable to conduct a scan in which a series of pulse
sequences are conducted in which the second magnetic field gradient
is stepped through a series of discrete values and a corresponding
series of NMR signals are sensed and digitized to form an NMR data
set; and
processor means for storing the NMR data set and for reconstructing
an image array for a display from the stored NMR data set by:
(a) Fourier transforming the NMR data set along one of its
dimensions to create hybrid-space data arrays I' and Q';
(b) producing a correction data array using the data in the
hybrid-space data arrays I' and Q';
(c) applying the data in the correction data array to the NMR data
set to reduce motion effects; and
(d) Fourier transforming the corrected NMR image data set to
produce the image array; and
in which the NMR data set acquired during the scan includes NMR
navigator data which has been subjected to one of said two magnetic
field gradients and NMR image data which has been subjected to both
of said two magnetic field gradients, and steps (a) and (b) are
performed with the NMR navigator data and step (c) is performed on
the NMR image data.
6. In an NMR system which performs a scan to acquire an NMR data
set from which an image array is reconstructed, the improvement
comprising:
transforming an NMR data set produced by the NMR system to create a
hybrid-space data array by performing a Fourier transformation on
the NMR data set along one of its dimensions;
producing a correction data array using the data in the
hybrid-space data array by calculating the magnitude of each
element of the hybrid-space data array to produce a corresponding
modulus array, and correlating each row of the modulus array to
produce a corresponding shift value for the correction data
array;
applying the data in the correction data array to an NMR data set
produced by the NMR system to reduce artifacts caused by motion in
the acquired NMR data set; and
producing an image array from the corrected NMR data set.
7. The improvement as recited in claim 6 in which a rollback
correction data array is produced from the corresponding shift
values in the correction data array and a rollover rate K.sub.R,
and the values in both the correction data array and the rollback
correction data array are applied to correct said acquired NMR data
set used to produce the image array.
8. In an NMR system which performs a scan to acquire an NMR data
set from which an image array is reconstructed, the improvement
comprising:
transforming an NMR data set produced by the NMR system to create a
hybrid-space data array by performing a Fourier transformation of
the NMR data set along one of its dimensions;
producing a correction data array using the data in the
hybrid-space data array by calculating the phase of each element of
the hybrid-space data array to produce the corresponding elements
of a two-dimensional phase data array, and determining the
difference in phase between elements in a reference row of the
phase data array and elements in the same column of the phase data
array;
applying the data in the correction data array to an NMR data set
produced by the NMR system to reduce artifacts caused by motion in
the acquired NMR data set; and
producing an image array from the corrected NMR data set.
9. The improvement as recited in claim 7 which a second correction
data array is produced by:
calculating the phase of each element of the hybrid-space data
array to produce the corresponding elements of a two-dimensional
phase data array having rows and columns; and
calculating the elements of the second correction data array by
determining the difference in phase between elements in a reference
row of the phase data array and elements in the same column of the
phase data array;
wherein the data in both the correction data array and the second
correction data array is applied to the acquired NMR data set to
reduce the effects of motion.
10. The improvement as recited in claim 6 in which the NMR data set
used to create the hybrid-space data array is acquired from a
navigator NMR signal which is produced during each pulse sequence
of a scan but which is not subjected to a changing magnetic field
gradient during the scan, and the NMR data set employed to produce
the image array is acquired from an NMR signal which is produced
during each pulse sequence of the scan and which is subject to a
phase encoding magnetic field gradient which changes during the
scan.
11. The improvement as recited in claim 10 in which each navigator
NMR signal is produced in the same pulse sequence with its
corresponding phase encoded NMR signal.
12. An NMR system, the combination comprising:
means for generating a polarizing magnetic field;
excitation means for generating an RF excitation magnetic field
which produces transverse magnetization in nuclei subjected to the
polarizing magnetic field;
receiver means for sensing the NMR signal produced by the
transverse magnetization and producing digitized in-phase (I) and
quadrature (Q) samples of the NMR signal;
first gradient means for generating a first magnetic field gradient
to impart a first phase component into the NMR signal which is
indicative of the location of the transversely magnetized nuclei
along a first coordinate axis;
second gradient means for generating a second magnetic field
gradient to impart a second phase component into the NMR signal
which is indicative of the location of the transversely magnetized
nuclei along a second coordinate axis;
pulse control means coupled to the excitation means, first and
second gradient means, and receiver means, said pulse control means
being operable to conduct a scan in which a series of pulse
sequences are conducted in which the second magnetic field gradient
is stepped through a series of discrete values and a corresponding
series of NMR signals are sensed and digitized to form an NMR data
set; and
processor means for storing the NMR data set and for reconstructing
an image array for a display from the stored NMR data set by:
(a) producing a phase array from the NMR data set which indicates
the phase of the digitized NMR signals;
(b) producing correction data using the data in the phase
array;
(c) applying the correction data to the NMR data set to reduce
motion effects; and
(d) Fourier transforming the corrected NMR image data set to
produce the image array.
13. An NMR system, the combination comprising:
means for generating a polarizing magnetic field;
excitation means for generating an RF excitation magnetic field
which produces transverse magnetization in nuclei subjected to the
polarizing magnetic field;
receiver means for sensing the NMR signal produced by the
transverse magnetization and producing digitized in-phase (I) and
quadrature (Q) samples of the NMR signal;
first gradient means for generating a first magnetic field gradient
to impart a first phase component into the NMR signal which is
indicative of the location of the transversely magnetized nuclei
along a first coordinate axis;
second gradient means for generating a second magnetic field
gradient to impart a second phase component into the NMR signal
which is indicative of the location of the transversely magnetized
nuclei along a second coordinate axis;
pulse control means coupled to the excitation means, first and
second gradient means, and receiver means, said pulse control means
being operable to conduct a scan in which a series of pulse
sequences are conducted in which the second magnetic field gradient
is stepped through a series of discrete values and a corresponding
series of NMR signals are sensed and digitized to form an NMR data
set; and
processor means for storing the NMR data set and for reconstructing
an image array for a display from the stored NMR data set by:
(a) producing a phase array from the NMR data set which indicates
the phase of the digitized NMR signals;
(b) producing an altered phase array by subtracting phase
components produced by stationary spins from each value in the
phase array;
(c) producing correction data using the data in the altered phase
array;
(d) applying the correction data to the NMR data set to reduce
motion effects; and
(e) transforming the corrected NMR image data set to produce the
image array.
14. The NMR system as recited in claim 13 in which the phase
components produced by stationary spin are calculated by
determining the center of a circle which best fits the values in
each column of the phase array.
Description
BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging
methods and systems. More particularly, the invention relates to a
method for reducing image artifacts caused by flow and motion.
Any nucleus which possesses a magnetic moment attempts to align
itself with the direction of the magnetic field in which it is
located. In doing so, however, the nucleus precesses around this
direction at a characteristic angular frequency (Larmor frequency)
which is dependent on the strength of the magnetic field and on the
properties of the specific nuclear species (the magnetogyric
constant .gamma. of the nucleus). Nuclei which exhibit this
phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform
magnetic field (polarizing field B.sub.z), the individual magnetic
moments of the spins in the tissue attempt to align with this
polarizing field, but precess about it in random order at their
characteristic Larmor frequency. A net magnetic moment M.sub.z is
produced in the direction of the polarizing field, but the randomly
oriented magnetic components in the perpendicular, or transverse,
plane (x-y plane) cancel one another. If, however, the substance,
or tissue, is subjected to a magnetic field (excitation field
B.sub.1) which is in the x-y plane and which is near the Larmor
frequency, the net aligned moment, M.sub.z, may be rotated, or
"tipped", into the z-y plane to produce a net transverse magnetic
moment M.sub.1, which is rotating, or spinning, in the x-y plane at
the Larmor frequency. The degree to which the net magnetic moment
M.sub.z is tipped, and hence the magnitude of the net transverse
magnetic moment M.sub.1 depends primarily on the length of time and
the magnitude of the applied excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which
is emitted by the excited spins after the excitation signal B.sub.1
is terminated. In simple systems the excited spin induce an
oscillating sine wave signal in a receiving coil. The frequency of
this signal is the Larmor frequency, and its initial amplitude,
A.sub.0, is determined by the magnitude of the transverse magnetic
moment M.sub.1. The amplitude, A, of the emission signal decays in
an exponential fashion with time, t:
The decay constant 1/T*.sub.2 depends on the homogeneity of the
magnetic field and on T.sub.2, which is referred to as the
"spin-spin relaxation" constant, or the "transverse relaxation"
constant. The T.sub.2 constant is inversely proportional to the
exponential rate at which the aligned precession of the spins would
dephase after removal of the excitation signal B.sub.1 in a
perfectly homogeneous field.
Another important factor which contributes to the amplitude A of
the NMR signal is referred to as the spin-lattice relaxation
process which is characterized by the time constant T.sub.1. This
is also called the longitudinal relaxation process as it describes
the recovery of the net magnetic moment M to its equilibrium value
along the axis of magnetic polarization (z). The T.sub.1 time
constant is longer than T.sub.2, much longer in most substances of
medical interest.
The NMR measurements of particular relevance to the present
invention are called "pulsed NMR measurements". Such NMR
measurements are divided into a period of excitation and a period
of signal emission. Such measurements are performed in a cyclic
manner in which the NMR measurement is repeated many times to
accumulate different data during each cycle or to make the same
measurement at different locations in the subject. A wide variety
of preparative excitation techniques are known which involve the
application of one or more excitation pulses (B.sub.1) of varying
magnitude and duration. Such excitation pulses may have a narrow
frequency spectrum (selective excitation pulse), or they may have a
broad frequency spectrum (nonselective excitation pulse) which
produces transverse magnetization M.sub.1 over a range of resonant
frequencies. The prior art is replete with excitation techniques
that are designed to take advantage of particular NMR phenomena and
which overcome particular problems in the NMR measurement process.
The present invention may be used with any of these pulse
sequences.
When utilizing NMR to produce images, a technique is employed to
obtain NMR signals from specific locations in the subject.
Typically, the region which is to be imaged (region of interest) is
scanned by a sequence of NMR measurement cycles which vary
according to the particular localization method being used. The
resulting set of received NMR signals are digitized and processed
to reconstruct the image using one of many well known
reconstruction techniques. To perform such a scan, it is, of
course, necessary to elicit NMR signals from specific locations in
the subject. This is accomplished by employing magnetic fields
(G.sub.x, G.sub.y, and G.sub.z) which have the same direction as
the polarizing field B.sub.0, but which have a gradient along the
respective x, y and z axes. By controlling the strength of these
gradients during each NMR cycle, the spatial distribution of spin
excitation can be controlled and the location of the resulting NMR
signals can be identified.
NMR data for constructing images can be collected using one of many
available techniques, such as multiple angle projection
reconstruction and Fourier transform (FT). Typically, such
techniques comprise a pulse sequence made up of a plurality of
sequentially implemented views. Each view may include one or more
NMR experiments, each of which comprises at least an RF excitation
pulse and a magnetic field gradient pulse to encode spatial
information into the resulting NMR signal. As is well known, the
NMR signal may be a free indication decay (FID) or, preferably, a
spin-echo signal.
The preferred embodiments of the invention will be described in
detail with reference to a variant of the well known FT technique,
which is frequently referred to as "spin-warp". The spin-warp
technique is discussed in an article entitled "Spin Warp NMR
Imaging and Applications to Human Whole-Body Imaging" by W. A.
Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp.
751-756 (1980).
Briefly, the spin-warp technique employs a variable amplitude phase
encoding magnetic field gradient pulse prior to the acquisition of
NMR spin-echo signals to phase encode spatial information in the
direction of this gradient. In a two-dimensional implementation
(2DFT), for example, spatial information is encoded in one
direction by applying a phase encoding gradient (G.sub.y) along
that direction, and then a spin-echo signal is acquired in the
presence of a read-out magnetic field gradient (G.sub.x) in a
direction orthogonal to the phase encoding direction. The read-out
gradient present during the spin-echo acquisition encodes spatial
information in the orthogonal direction. In a typical 2DFT pulse
sequence, the magnitude of the phase encoding gradient pulse
G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views
that are acquired during the scan to produce a set of NMR data from
which an entire image can be reconstructed. Object motion during
the acquisition of NMR image data produces both blurring and
"ghosts" in the phase-encoded direction. Ghosts are particularly
apparent when the motion is periodic, or nearly so. For most
physiological motion each view of the NMR signal is acquired in a
period short enough that the object may be considered stationary
during the acquisition window. In such case the blurring and
ghosting is due to the inconsistent appearance of the object from
view to view. Motion that changes the appearance between views such
as that produced by a patient moving, by the respiration or the
cardiac cycle, or by peristalsis, is referred to hereinafter as
"view-to-view motion". Motion may also change the amplitude and
phase of the NMR signal as it evolves during the pulse sequence and
such motion is referred to hereinafter as "in-view motion".
Both blurring and ghosting can be reduced if the data acquisition
is synchronized with the functional cycle of the object to reduce
view-to-view motion. This method is known as gated NMR scanning,
and its objective is to acquire NMR data at the same point during
successive functional cycles so that the object "looks" the same in
each view. The drawback of gating is that NMR data may be acquired
only during a small fraction of the object's functional cycle, and
even when the shortest acceptable pulse sequence is employed, the
gating technique can significantly lengthen the data acquisition
time. Some of these methods are disclosed in U.S. Pat. Nos.
4,751,462; 4,567,893 and 4,663,591. None of them have proven
entirely satisfactory because they either depend upon perfectly
periodic motion, or they increase the scan time significantly, or
they produce low signal-to-noise images.
Several NMR pulse sequences have been proposed to either
desensitize the NMR measurement to the phase perturbations caused
by flowing spins as described in U.S. Pat. No. 4,728,890, or to
sensitize it to flow in such a manner that the effects of flow can
properly be separated from the reconstructed images as described in
U.S. Pat. No. RE 32,701. None of these methods have proven entirely
satisfactory, either from a performance standpoint, or because of
their adverse impact on scan time or the type of NMR measurements
that may be performed.
In our prior U.S. Pat. No. 4,715,383, we disclose a method for
reducing motion and flow artifacts in NMR images. While this method
substantially improves NMR images by suppressing artifacts caused
by spins outside the region of interest, it does not correct for
the motion artifacts produced by spins located inside the region of
interest.
All of the prior methods for reducing motion and flow artifacts
focus on the data acquisition procedure. They change the NMR pulse
sequence itself, they change the order in which the pulse sequences
in a scan are executed, or they synchronize the execution of the
pulse sequence with the motion of the subject under study. Their
objective is to produce a set of NMR data which is minimally
affected by flow and motion and which can, therefore, be used to
construct a clear, ghost-free image.
SUMMARY OF THE INVENTION
The present invention is a method for reducing motion and flow
artifacts in an NMR image by correcting the set of NMR data which
has been acquired during a scan to remove the effects of motion and
flow before the image is reconstructed. More specifically, the
invention includes transforming the NMR data set to create a
hybrid-space data array; producing a correction data array using
the data in the hybrid-space data array; and applying the data in
the correction data array to the NMR data set produced by the NMR
system to reduce flow and motion artifacts in the image which is
reconstructed from the NMR data set. The correction data array
which is produced in accordance with one aspect of the present
invention corrects for view-to-view motion artifacts and the
correction data array which is produced in accordance with another
aspect of the invention corrects for in-view motion and flow
artifacts.
A general object of the invention is to provide a motion and flow
artifact correction method which can be employed after the NMR data
has been acquired. The present invention enables the NMR data set
to be corrected retrospective of its acquisition and, therefore, it
may be used in addition to any motion or flow artifact suppression
techniques which have been employed in the past. To the extent that
the acquired NMR data is affected by motion or flow, the present
invention will detect it and automatically correct the NMR data so
that the affect is reduced or eliminated.
Another aspect of the invention is to acquire "navigator" NMR data
along with the usual image NMR data within the same pulse sequence.
The navigator NMR data enables the corrections for view-to-view and
in-view motion artifacts to be made more accurately. A navigator
NMR signal is produced in each pulse sequence along with the image
NMR signal and a data set is acquired for both. The corrective
values are determined using the navigator data set and the
corrections are made to the image data set.
A more specific object of the invention is to correct the NMR data
set for view-to-view artifacts caused by motion or flow in any
direction. The navigator signal can be acquired in the presence of
a read-out magnetic field gradient which is oriented in any
direction. The shift corrections which are produced according to
the present invention, will correct for errors caused by
view-to-view motion or flow in the direction of the navigator
signal read-out gradient By acquiring more than one navigator
signal in the pulse sequence in the presence of read-out gradients
oriented in respective different directions, shift corrections are
produced which correct for view-to-view motion in the corresponding
directions. For example, shift corrections can be made for motion
along both the x axis and the y axis of the NMR system.
Yet another specific object of the invention is to correct the NMR
data set for in-view artifacts caused by motion or flow in any
direction. The phase corrections P which are produced according to
the present invention offset artifact causing systematic noise
produced by in-view motion or flow regardless of its direction.
Still another aspect of the present invention is that both in-view
and view-to-view motion and flow artifact correction can be applied
to the same NMR data set. Both the in-view correction data array
and the view-to-view correction data array can be applied to the
NMR data set.
The foregoing and other objects and advantages of the invention
will appear from the following description. In the description,
reference is made to the accompanying drawings which form a part
hereof, and in which there is shown by way of illustration a
preferred embodiment of the invention. Such embodiment does not
necessarily represent the full scope of the invention, however, and
reference is made therefore to the claims herein for interpreting
the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an NMR system which employs the
present invention;
FIG. 2 is an electrical block diagram of the transceiver which
forms part of the NMR system of FIG. 1;
FIG. 3 is a graphic representation of a conventional NMR pulse
sequence used to acquire data to produce an image;
FIG. 4 is a pictorial representation of how an image is
reconstructed from NMR data acquired using the pulse sequence of
FIG. 3;
FIG. 5 is a pictorial representation of how correction values are
calculated according to the present invention;
FIG. 6 is a graphic representation of data in the modulus array of
FIG. 5;
FIG. 7 is a graphic representation of the correlation process used
to produce the shift correction array of FIG. 5;
FIGS. 8a and 8b are graphic representations of data in the phase
array of FIG. 5 and the process used to produce the phase
correction array of FIG. 5;
FIG. 9 is a graphic representation of an alternative pulse sequence
used to acquire both image NMR data and navigator NMR data; and
FIG. 10 is a plot of hybrid-space data which illustrates the
effects of both stationary and moving spins on hybrid-space
phase.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown in block diagram form the major
components of a preferred NMR system which incorporates the present
invention and which is sold by the General Electric Company under
the trademark "SIGNA". The overall operation of the system is under
the control of a host computer system generally designated 100
which includes a main computer 101 (a Data General MV4000). The
computer 100 includes an interface 102 through which a plurality of
computer peripheral devices and other NMR system components are
coupled to the main computer 101. Among the computer peripheral
devices is a magnetic tape drive 104 which may be utilized under
the direction of the main computer 101 for archiving patient data
and image data to tape. Processed patient data may also be stored
in an image disc storage device designated 110. An array processor
106 is utilized for preprocessing acquired NMR data and for image
reconstruction. The function of image processor 108 is to provide
interactive image display manipulation such as magnification, image
comparison, grayscale adjustment and real time data display. The
computer system 100 also includes a means to store raw NMR data
(i.e. before image construction) which employs a disc data storage
system designated 112. An operator console 116 is also coupled to
the main computer 101 by means of interface 102, and it provides
the operator with the means to input data pertinent to a patient
study as well as additional data necessary for proper NMR system
operation, such as calibrating, initiating and terminating scans.
The operator console is also used to display images stored on disc
or magnetic tape.
The computer system 100 exercises control over the NMR system by
means of a system control 118 and a gradient amplifier system 128.
Under the direction of a stored program, the computer 100
communicates with system control 118 by means of a serial
communication network 103 (such as the Ethernet network) in a
manner well known to those skilled in the art. The system control
118 includes several subsystems such as a pulse control module
(PCM) 120, a radio frequency transceiver 22, a status control
module (SCM) 124, and power supplies generally designated 126. The
PCM 120 utilizes control signals generated under program control by
main computer 101 to generate digital waveforms which control
gradient coil excitation, as well as RF envelope waveforms utilized
in the transceiver 122 for modulating the RF excitation pulses. The
gradient waveforms are applied to the gradient amplifier system 128
which is comprised of G.sub.x, G.sub.y and G.sub.z amplifiers 130,
132 and 134, respectively. Each amplifier 130, 132 and 134 is
utilized to excite a corresponding gradient coil in an assembly
designated 136 which is part of a magnet assembly 146. When
energized, the gradient coils generate magnetic field gradients
G.sub.x, G.sub.y and G.sub.z.
The gradient magnetic fields are utilized in combination with radio
frequency pulses generated by transceiver 122, RF amp 123 and RF
coil 138 to encode spatial information into the NMR signals
emanating from the region of the patient being studied. Waveforms
and control signals provided by the pulse control module 120 are
utilized by the transceiver subsystem 122 for RF carrier modulation
and mode control. In the transmit mode, the transmitter provides a
radio frequency signal to an RF power amplifier 123 which then
energizes RF coils 138 which are situated within main magnet
assembly 146. The NMR signals radiated by the excited spin in the
patient are sensed by the same or a different RF coil than is used
for transmitting. The signals are detected, amplified, demodulated,
filtered, and digitized in the receiver section of the transceiver
122. The processed signals are transmitted to the main computer 101
by means of a dedicated, unidirectional, high-speed digital link
105 which links interface 102 and transceiver 122.
The PCM 120 and SCM 124 are independent subsystems both of which
communicate with main computer 101, peripheral systems, such as
patient positioning system 152, as well as to one another by means
of serial communications link 103. The PCM 120 and SCM 124 are each
comprised of a 16-bit microprocessor (such as Intel 8086) for
processing commands from the main computer 101. The SCM 124
includes means for acquiring information regarding patient cradle
position, and the position of the moveable patient alignment light
fan beam (not shown). This information is used by main computer 101
to modify image display and reconstruction parameters. The SCM 124
also initiates functions such as actuation of the patient transport
and alignment systems.
The gradient coil assembly 136 and the RF transmit and receiver
coils 138 are mounted within the bore of the magnet utilized to
produce the polarizing magnetic field. The magnet forms a part of
the main magnet assembly which includes the patient alignment
system 148, a shim coil power supply 140, and a main magnet power
supply 142. The main power supply 412 is utilized to bring the
polarizing field produced by the magnet to the proper operating
strength of 1.5 Tesla and is then disconnected.
To minimize interference from external sources, the NMR system
components comprised of the main magnet assembly, the gradient coil
assembly, and the RF transmit and receiver coils, as well as the
patient-handling devices, are enclosed in an RF shielded room
generally designated 144. The shielding is generally provided by a
copper or aluminum screen network which encloses the entire room.
The screen network serves to contain the RF signals generated by
the system, while shielding the system from RF signals generated
outside the room.
Referring particularly to FIGS. 1 and 2, the transceiver 122
includes components which produce the RF excitation field B.sub.1
through power amplifier 123 at a coil 138A and components which
receive the resulting NMR signal induced in a coil 138B. The base,
or carrier, frequency of the RF excitation field is produced by a
frequency synthesizer 200 which receives a set of digital signals
through the communications link 103 from the main computer 101.
These digital signals indicate the frequency which is to be
produced at an output 201 at a resolution of one Hertz. This
commanded RF carrier is applied to a modulator 202 where it is
frequency and amplitude modulated in response to signals received
through line 203, and the resulting RF excitation signal is turned
on and off in response to a control signal which is received from
the PCM 120 through line 204. The magnitude of the RF excitation
pulse output through line 205 is attenuated by a transmit
attenuator circuit 206 which receives a digital signal from the
main computer 101 through communications link 103. The attenuated
RF excitation pulses are applied to the power amplifier 123 that
drives the RF transmitter coil 138A.
Referring still to FIGS. 1 and 2, the NMR signal produced by the
excited spin in the subject is picked up by the receiver coil 138B
and applied to the input of a receiver 207. The receiver 207
amplifies the NMR signal and this is attenuated by an amount
determined by a digital attenuation signal received from the main
computer 101 through link 103. The receiver 207 is also turned on
and off by a signal through line 208 from the PCM 120 such that the
NMR signal is acquired only over the time intervals required by the
particular acquisition being performed.
The received NMR signal is demodulated by a quadrature detector 209
to produce two signals I and Q that are coupled through
anti-aliasing filters 216 and 217 to a pair of analog to digital
converters indicated collectively at 218. The quadrature detector
209 also receives an RF reference signal from a second frequency
synthesizer 210 and this is employed by the quadrature detector 209
to sense the amplitude of that component of the NMR signal which is
in phase with the transmitter RF carrier (I signal) and the
amplitude of that component of the NMR signal which is in
quadrature therewith (Q signal).
The I and Q components of the received NMR signal are continuously
sampled and digitized by the A/D converter 218 at a sample rate of
64 kHz throughout the acquisition period. A set of 256 digital
numbers are acquired for each I and Q component of the NMR signal,
and these digital numbers are conveyed to the main computer 101
through the serial link 105.
The NMR system of FIG. 1 performs a series of pulse sequences to
collect sufficient NMR data to reconstruct an image. One such pulse
sequence is shown in FIG. 3. This sequence performs a slice
selection by applying a 90.degree. selective RF excitation pulse
300 in the presence of a z axis gradient pulse 301 and its
associated rephasing pulse 302. After an interval TE.sub.1 /2, a
180.degree. selective RF excitation pulse 303 is applied in the
presence of another z axis gradient pulse 304 to refocus the
transverse magnetization at the time TE.sub.1 and produce an echo
NMR signal 305.
To position encode the echo NMR signal 305, an x axis read-out
gradient pulse 306 is applied during the acquisition of the NMR
signal 305. The read-out gradient frequency encodes the NMR signal
305 in the well known manner. In addition, the echo NMR signal 305
is position encoded along the y axis by a phase encoding gradient
pulse 307. The phase encoding gradient pulse 307 has one strength
during each echo pulse sequence and associated NMR echo signal 305,
and it is typically incremented in steps through 256 discrete
strengths (-128 to +128) during the entire scan. As a result, each
of the 256 NMR echo signals 305 acquired during the scan is
uniquely phase encoded.
It is, of course, usual practice to repeat the pulse sequence for
each phase encoding gradient value one or more times and to combine
the acquired NMR signals in some manner to improve signal-to-noise
and to offset irregularities in the magnetic fields. In the
following discussion, it is assumed that such techniques may be
used to acquire the NMR data set which is to be corrected.
Referring particularly to FIG. 4, the acquired NMR data is stored
in the data disk 112 (FIG. 1) in the form of two 256.times.256
element arrays 310 and 311. The array 310 contains the in-phase
magnitude values I and the array 311 contains the quadrature values
Q. Together these arrays 310 and 311 form an NMR image data set
which defines the acquired image in what is referred to in the art
as "k-space".
To convert this k-space NMR data set into data which defines the
image in real space (i.e. Cartesian coordinates), a two step
Fourier transformation is performed on the I and Q arrays 310 and
311. The transformation is performed first in the read-out
direction which is the horizontal rows of the arrays 310 and 311 to
produce two 256.times.256 element arrays 312 and 313. The array 312
contains the in-phase data and is labeled I', while the array 313
contains the quadrature data and is labeled Q'. The I' and Q'
arrays 312 and 313 define the acquired image in what is referred to
in the art as "hybrid-space". This first transformation of the
acquired NMR data set is expressed mathematically as follows:
##EQU1##
The second transformation is performed in the phase encoding
direction which is the vertical columns of the arrays 312 and 313
to produce two 256.times.256 element arrays 314 and 315 The array
314 contains the transformed in-phase values and is labeled I",
while the array 315 contains the quadrature values and is labeled
Q". This second transformation may be expressed mathematically as
follows: ##EQU2## The arrays 314 and 315 are a data set which
defines the acquired image in real space and the elements thereof
are used to calculate the intensity values in a 256.times.256
element image array 316 in accordance with the following
expression: ##EQU3## The 256.times.256 elements of the image array
316 are mapped to the main operator console 116 (FIG. 1) for
display on a CRT screen.
The above described NMR system and pulse sequence is representative
of the current state of the art. The diagnostic quality of the
image which is obtained is determined by the extent to which the
acquired NMR signals are degraded by superimposed thermal
(statistical) noise and systematic (artifact) noise (which includes
unsharpness). The sources of thermal noise are well understood. The
ratio of NMR signal intensity to thermal noise is determined by
such factors as polarizing magnetic field strength, RF receiver
coil configuration, the particular pulse sequence used, and the
amount of signal averaging used. Systematic noise, mainly resulting
from physiological motion, may degrade the NMR image far beyond the
fundamental limit set by the thermal noise. Indeed, the diagnostic
quality of many NMR images is limited far more by motion artifact
and other forms of systematic noise than by intrinsic thermal
noise. However, in contrast to thermal noise, there is no inherent
limit on the extent to which systematic noise can be reduced.
It is the reduction of systematic noise and unsharpness which the
present invention addresses. Whereas prior techniques address this
problem with improvements to the NMR system hardware (for example,
cardiac and respiratory gating circuits) or with improvements to
the pulse sequence (for example, motion desensitizing gradient
pulses), the present invention typically addresses the problem
retrospectively. More specifically, it is a discovery of the
present invention that systematic noise can be easily detected by
examining the NMR data set in hybrid-space. Systematic noise caused
by view-to-view motion as well as systematic noise caused by
in-view motion can be detected. Corrective operators can then be
calculated and used to eliminate the systematic noise from the NMR
image data set. The usual image reconstruction process can then be
performed using the corrected NMR image data set to produce an
image which is substantially free of artifacts caused by
motion.
Referring to FIG. 5, the first step is to produce the hybrid-space
I' and Q' arrays 312 and 313 as described above (equation 1). The
image data set itself can be used to produce the hybrid-space
arrays 312 and 313, or as will be described in more detail below,
separate NMR data produced by a navigator NMR signal within the
same pulse sequence can be used. In either case, corrective values
for both view-to-view and in-view systematic noise can be
calculated from the hybrid-space arrays.
Referring to FIGS. 5 and 6, it has been discovered that
view-to-view systematic noise can be detected in the magnitude
information contained in the hybrid-space arrays 312 and 313.
Accordingly, the next step in the process is to produce a
256.times.256 modulus array (M) 320. This is accomplished by
calculating each element M.sub.xy of the modulus array 320 from the
corresponding elements I'.sub.xy and Q'.sub.xy of the hybrid-space
arrays 312 and 313. ##EQU4## where: x=array column number, 1-256;
and
y=array row number, 1-256.
It is a discovery of the present invention that view-to-view motion
in the direction of the associated read-out gradient can be seen as
a shifting left or right of the modulus data in each row of the
modulus array 320. This is illustrated in FIG. 6, where each graph
is the modulus values in a horizontal row of the modulus array 320
plotted as a function of its column number in the array 320.
Although each graph is slightly different due to the differences in
the amount of phase encoding, the graphs do have significant peaks
which should occur at the same column number of the array 320. This
is illustrated by the dashed lines 325 and 326 through the peaks
327 and 328 in the graph of the first row (y=1). The corresponding
peaks 327' and 328' in the graph of the second row (y=2) are
shifted to the left of the dashed lines 325 and 326, and the
corresponding peaks 327" and 328" in the last row of data (y=256)
are shifted to the right. Examination of the remaining 509 rows of
the modulus array 320 would show similar shifts in varying amounts
which are caused by view-to-view movement of the subject.
The next step in the process is to determine the amount, S, by
which each row of modulus data must be shifted to bring it into
correlation with a reference row. This cross correlation may be
performed in a number of ways. In the preferred embodiment a
reference row (M.sub.r) in the modulus array 320 is selected and it
is cross correlated with each of the other 255 rows M.sub.y. This
is done by determining how much each row M.sub.y must be shifted
left or right to maximize the sum of the product of its elements
and the corresponding elements in the reference row M.sub.r.
ln other words, the following expression is calculated and stored:
##EQU5##
The data on row M.sub.y is then shifted one position and the
process is repeated These calculations are performed for M.sub.y
shifted between -32 and +32 positions, although these can be
extended if necessary for larger displacements. A plot of an
exemplary correlation curve which results from these sixty-four
calculations is shown in FIG. 7. It can be seen that the peak in
this exemplary curve occurs when the row of modulus data M.sub.y is
shifted to the left a few positions. Accordingly, a corrective
value S.sub.y is determined by finding the number of shifts needed
to produce the peak in the correlation curve. A shift correction
value S.sub.y is calculated for each row (y=1 to 256) of the
modulus array 320 and is stored in a 1.times.256 element shift
correction array 330. There are many other ways to correlate each
row in the modulus array 320.
A very significant reduction in view-to-view motion in the x axis
can be achieved by applying the correction values S directly to the
modulus array 320 and reconstructing an image from this corrected
NMR data. This is accomplished by shifting the data in each row of
the modulus 320 by the amount indicated by the corresponding
element of the shift correction array 330. Then, the values in the
I' and Q' hybrid-space arrays 312 and 313 are calculated using the
corrected modulus values and the known phase angle for each
element. An image can then be reconstructed in the usual fashion
from the corrected I' and Q' hybrid-space arrays 312 and 313 as
described above.
While the above-described correction procedure provides significant
improvement in image quality, further improvements can be made.
When the shift correction S is made, a small amount of phase error
is introduced into the NMR data set. This is due to the phase
rollover which occurs in any NMR system as a result of asymmetry in
the echo signal in its acquisition window. The signal represented
by the elements in each column of the I' and Q' hybrid arrays 312
and 313 contain the same amount of rollover phase, and the amount
of rollover phase changes linearly at the rollover rate K.sub.R as
the arrays are traversed from left to right through its columns
(x=1 to 256). Thus, when data in a row is shifted left or right to
make the S correction discussed above, the data moves into a column
with a different phase value than the column of its origin. This
rollover phase can be corrected and is a function of the amount
which the data was shifted:
The rollback correction values for each row in hybrid-space is
calculated to produce a 1.times.256 element rollback correction
array 331. Each element in the array 331 indicates the phase
correction which must be made to all elements in its corresponding
row of the I' and Q' hybrid-space arrays 312 and 313. How these
phase corrections are actually made will be discussed in more
detail below.
The correction made thus far accounts for view-to-view motion along
the read-out x axis. As will be discussed in more detail below,
similar corrections can be made along the phase encoding y axis by
using a special navigator signal in the same pulse sequence as the
image signal and which is acquired in the presence of a y axis
read-out gradient pulse.
The above-described corrections do not account for phase errors in
the NMR data due to in-view motion or flow. Such errors occur
because the spins are moving during the pulse sequence. It is a
further discovery of the present invention that these phase errors
can be detected in the k-space or the hybrid-space NMR data
set.
Referring to FIGS. 5 and 8, the procedure begins again with the I'
and Q' hybrid-space arrays 312 and 313. This data set is used to
calculate the phase .PHI. of the acquired NMR signal at each of the
256 sample times and at each of the 256 views. A 256 by 256 element
phase array 335 is produced, with each of its elements having a
value which is calculated as follows:
It is a discovery of the present invention that the calculated
phase .PHI. should have the same value along any column (y=1 to
256) of the phase array 335. This is shown graphically in FIG. 8a
where the calculated phase .PHI. for three rows of data has been
plotted as a function of column number (x=1 to 256). For any given
NMR data set which is produced without phase encoding gradients
(i.e. a navigator signal), these plots will be substantially the
same if there are no phase errors. To the extent that the
calculated phase .PHI. differs in any column, that difference
represents a phase error which should be corrected. This is
graphically illustrated in FIG. 8b where two of the phase plots in
FIG. 8a are superimposed on one another to reveal differences in
their values over the central region. These differences are
illustrated by the arrows .PHI..sub.1 and .PHI..sub.2.
To correct for the phase error, therefore, a 256 by 256 element
phase correction array 337 is produced. This is accomplished by
establishing one row (y=1) of phase data in the .PHI. array 335 as
a reference and then finding the difference between the value of
one of its elements and the same element o(in each of the other
rows (y=2 to 256) of the .PHI. array 335. This is repeated for each
element (x=1 to 256) of the reference row (y=1) and the calculated
difference values (.DELTA..PHI..sub.P) are stored in the
corresponding location in the phase correction array 337. Thus,
each value in the phase correction array 337 indicates the amount
by which the phase of each element of the NMR data set should be
corrected. This corrects for in-view motion in any direction (x, y
or z). How this correction is actually made will be described in
more detail below.
While separate phase correction values .DELTA..PHI..sub.P are
calculated for each of the 256 by 256 elements of the array 337 in
the preferred embodiment, it should be apparent that a less
rigorous approach can also be used. If the in-view motion is known
to occur over only a small segment of the x axis field of view,
then the calculation of corrective values .DELTA..PHI..sub.P may be
limited to that segment. Furthermore, if the in-view motion is
uniform over that x axis segment, then perhaps a single value
.DELTA..PHI..sub.P will suffice as a correction over the entire
segment. Since these corrections are applied retrospectively to the
acquired NMR data set, it is contemplated that correction variables
such as this will be under operator control and the radiologist can
manipulate the corrections to obtain the image he needs in minimal
time.
When calculating the phase corrections which are to be made, one
must consider whether both moving and stationary spins are
contributing to the measured phase values. Where all the spins are
moving, no further correction is necessary. However, when
stationary spins are contributing substantial signal, then the
phase correction values can be further refined.
The effects of NMR signal components produced by both stationary
and moving spins is illustrated in FIG. 10. The points on this plot
represent the I' and Q' values for the 256 data samples in a single
column of the hybrid-space data sets 312 and 313. These points
define a circle which is illustrated by dashed line 401 having a
center 402. If no motion were present, all of the points in this
ring would cluster at one location. On the other hand, if all the
spins are in motion, then the ring 401 would be formed, but its
center 402 would be at the origin (Q'=0, I'=0). In the illustrated
example, the signal components due to stationary spins are
represented by the vector A which offsets the center 402 from the
origin. The signal component due to moving spins is represented by
the vector B and its magnitude determines the size of the circle
401. The values (.PHI.) in the phase array 335 discussed above
indicate the phase angle of the combined stationary and moving
spins. To provide a more accurate correction for in-view motion,
therefore, the values in the phase array 335 should be altered to
indicate the phase .PHI.' of only the moving spins.
This alteration of the phase array values is performed on one
column at a time. First, the 256 data points in the column of the
hybrid-space arrays 312 and 313 are applied to a curve fitting
program which determines the center 402 of the circle 401. The
coordinate values of the center 402 are then subtracted from the
values in that column of the respective hybrid-space arrays 312 and
313. These altered values are then used to produce the altered
phase array 335, which, in turn, is used to produce the more
accurate phase correction array 337 as described above.
While the phase error detection method described above works well
when applied to a hybrid-space phase array which is derived from
data acquired from navigator NMR signals, less rigorous phase error
detection methods are also possible using the NMR image data set
itself. For example, the I and Q arrays 310 and 311 can be used to
calculate a 256.times.256 phase array in k-space. While the phase
values in this array may not be useful at its outer boundaries, the
phase information at the center column (x=128) where the peak of
the NMR echo signal 305 is sampled is usually unambiguous. One
element in this column is selected as the reference and all the
other values are compared with it to produce a 1.times.256 element
phase correction array. These correction values .DELTA..PHI..sub.P
may be applied to the NMR data set as will now be described.
All of the corrections calculated according to the present
invention can be made to the NMR raw image data set I and Q in
k-space. In k-space all of the corrections are implemented as a
rotation of the phase of each NMR signal sample (S.sub.xy =I.sub.xy
+jQ.sub.xy). The corrected NMR data set S'.sub.xy may thus be
calculated as follows:
This rotation is implemented in the I and Q arrays 310 and 311 in
accordance with the following expressions:
The phase change .DELTA..PHI..sub.T is the arithmetic sum of the
phase changes needed to make all of the above-described
corrections. Specifically, the total phase correction
.DELTA..PHI..sub.T is calculated as follows:
This expression includes the rollback correction .DELTA..PHI..sub.R
from the array 331 (FIG. 5) and the phase correction
.DELTA..PHI..sub.P from the array 337 (FIG. 5). The phase
correction .DELTA..PHI..sub.P is multiplied by a conversion factor
K, however, to account for any difference in phase shifts between
the image NMR signal and a navigator NMR signal which may be used
to determine the phase shift correction. If the image NMR data set
is used to calculate the phase corrections, this conversion factor
is "one". Otherwise, the value of K is measured by comparing the
phase shift produced in image data (with phase encoding gradient
applied) and the phase shift produced in the navigator data. In the
alternative, the value of K can be calculated.
The phase rotations .DELTA..PHI..sub.x and .DELTA..PHI..sub.y are
made to correct for view-to-view motion and flow effects along the
respective x and y axes. These phase corrections are calculated
from the shift values S in the shift correction array 330 (FIG.
5):
where:
S.sub.x =shift correction from array 330
x=sample number (i.e. 0 through 255)
N.sub.x =total number of samples during read-out (256).
where:
S.sub.y =shift correction calculated from navigator signal acquired
in presence of a y axis magnetic field gradient
y=phase encoding view number (0 to 255)
N.sub.y =total number of phase encoding views (256).
It should be apparent that the correction is different for each
element of the NMR image data arrays I and Q because the total
phase correction .DELTA..PHI..sub.T is different for each x and y
location in these arrays.
As indicated above, the corrective methods of the present invention
are applied to the NMR data set which has been acquired for the
purpose of producing the desired image. While this same image data
set can also be used to derive the corrective values which are to
be applied to it, an alternative procedure is to produce a separate
NMR data set during the same acquisition scan from which the
corrective values can more accurately be derived. This separate NMR
data set is produced by acquiring one or more "navigator" NMR
signals during each pulse sequence. The main distinction of these
navigator NMR signals is that they are not subject to the
application of incremented phase encoding gradients. In some cases,
a constant phase encode gradient may be used to elicit
two-dimensional investigatory information.
A navigator pulse sequence is shown in FIG. 9. As with the
conventional pulse sequence of FIG. 3, the spins are excited by a
90.degree. selective RF excitation pulse 300 in the presence of
slice select gradient pulses 301 and 302, and at a time interval
TE.sub.1 /2 later, the spins are subjected to a 180.degree.
selective RF excitation pulse 303 in the presence of slice select
gradient pulse 304. No phase encoding gradients are applied,
however, and at interval TE.sub.1 an NMR echo signal 340 is
acquired in the presence of x axis read-out gradient pulse 306. A
second 180.degree. RF excitation pulse 341 is then produced in the
presence of a slice select gradient pulse 342, and the y axis phase
encoding pulse 307 is then applied. The resulting NMR echo signal
305 produced at TE.sub.2 is then acquired in the presence of
read-out gradient pulse 306' to build the NMR image data set as
described above.
The NMR data set which is created by acquiring the NMR navigator
signal 340 is subject to substantially the same motion effects as
the NMR image data set which is acquired at the same time. However,
because the navigator signals 340 are not phase encoded, the
magnitude of the navigator NMR signals 340 are not significantly
diminished at the extremities of the field of view. Corrective
values can, therefore, be more readily calculated using the NMR
navigator signal data set rather than the NMR image data set. The
corrective values described above are thus determined by using the
acquired NMR navigator signal data set, and these corrective values
are then applied to the NMR image data set. The corrected NMR image
data set is then used to reconstruct an image.
Referring again to FIG. 5, the NMR navigator signal data set is
transformed first along the read-out gradient direction to produce
the hybrid-space I' and Q' arrays 312 and 313. This data is then
used as described above to produce a shift correction array 330, a
rollback correction array 331 and a phase correction array 337.
These corrective values are applied to the NMR image data set 310
and 311 (FIG. 4) as described above, and the corrected NMR image
data set is processed in the usual manner to produce the corrected
image array 316 (FIG. 4).
The shift corrections S correct for view-to-view motion in the
direction of the read-out gradient. Accordingly, the NMR navigator
signal 340 in FIG. 9 is acquired in the presence of the read-out
gradient pulse 306 which is directed along the x axis and the shift
corrections will compensate for motion along the x axis.
Alternatively, the NMR navigator signal 340 can also be acquired in
the presence of a read-out gradient pulse which is directed along
the phase encoding, or y axis, as shown by the dashed lines 343. In
yet another alternative, the pulse sequence can be modified to
produce two NMR navigator signals, with one being acquired during
an x axis read-out gradient pulse and the other being acquired in
the presence of a y axis read-out gradient pulse. Either one of the
navigator signal NMR data sets can then be used to produce the
phase correction array 337 (FIG. 5), while both navigator signal
NMR data sets are used to produce corresponding shift correction
arrays 330. A rollback correction array 331 is not always required
for shift corrections made in the phase encoding direction, y. Both
the x and y shift corrections can be made in hybrid-space, or they
may be converted to phase angle corrections which are made to the
NMR image data set in k-space as described above.
The two-dimensional shift correction data can also be derived using
a single navigator NMR signal acquired during the application of a
read-out gradient. As previously described, the magnitude of the
navigator NMR signal yields the correction values S along the
frequency encoding direction (x). In addition, however, the phase
of this navigator NMR signal can be used to calculate the
correction values along the phase encoding direction (y) as
follows: ##EQU6## where: .DELTA..PHI..sub.py is the measured phase
shift, A.sub.y is the amplitude of the applied phase encoding
gradient, and N.sub.Y is the number of phase encoding increments
(i.e. views).
The correction values for use in the above equation (10) for the y
direction is, therefore, given by the following when N.sub.y is
(256): ##EQU7## where: j is the view number.
While the navigator NMR signal is an echo signal in the preferred
embodiment, this is not a necessary requirement. The navigator NMR
signal may also be produced as a gradient recalled signal or as the
result of three-dimensional data acquisition pulse sequence.
The preferred embodiment of the above-described invention is
implemented by a Fortran program which is executed by the main
computer 101 for correcting in-view motion and view-to-view motion
in the direction of the x axis read-out gradient.
While the corrective values which are derived according to the
present invention are employed retrospectively to improve image
quality, they can also be employed to alter the scan. This is
particularly true of the shift values S which correct for
view-to-view motion. More specifically, as each pulse sequence is
executed and a row of raw NMR data is acquired for the image arrays
310 and 311, the row of data may be transformed to hybrid-space
(equation (1)), the modulus values calculated (equation (4)), and a
correlation made with the previously acquired row of NMR data
(equation (5)). The resulting shift value S can then be used to
alter the operating conditions of the NMR system before the next
pulse sequence is executed. For example, where the shift value
corrects for motion in the y axis direction, the phase of the RF
carrier signal produced by the transmit frequency synthesizer 200
(FIG. 2) is changed by the amount -.DELTA..PHI..sub.y (equation
(9)). Similarly, a shift correction for motion along the slice
select direction (z axis) can be made by changing the frequency of
the RF carrier produced by the transmit frequency synthesizer 200.
Shift corrections for motion along the read-out direction (x axis)
can be made by changing the location of the field of view along the
x axis.
The present invention addresses the problems currently associated
with magnetic resonance angiography. In addition, this invention is
necessary for quantitative tissue characterization using magnetic
resonance. The invention is not limited to Fourier image
reconstructions (i.e. 2DFT or 3DFT) and may be used with line
scanning and other projection reconstructions.
* * * * *